1. Field of the Invention
The present invention relates to the field of mobile communications. More particularly, the present invention relates to a mobile communication apparatus including an antenna array, wherein the mobile communication apparatus is capable of minimizing the effect of fading, interference and noise, and a mobile communication method used in the mobile communication apparatus.
2. Description of the Related Art
A next-generation mobile communication system can transmit information faster than a current PCS mobile communication system. Europe and Japan have adopted a wideband code division multiple access (W-CDMA) system, which is an asynchronous system, as a wireless access standard, while North America has adopted a CDMA-2000 (multi-carrier code division multiple access) system, which is a synchronous system.
In a conventional mobile communication system, several mobile stations communicate with one another through a base station. In order to transmit information at a high rate of speed, a mobile communication system should minimize loss due to characteristics of a mobile communication channel, such as fading and user interference. Diversity systems are used to prevent communication from becoming unstable due to fading. For example, a space diversity system, which is a type of diversity system, uses multiple antennas.
Since the use of multiple antennas can minimize the interference between users, a next-generation mobile communication system should use multiple antennas. Among diversity systems that overcome fading using multiple antennas, a multiple transmitting antenna system, used to increase the capacity of a transmission terminal, requires significant bandwidth in a transmission direction in view of the characteristics of next-generation mobile communication.
In order to achieve fast information transmission, a conventional mobile communication system should overcome fading, which is one of the channel characteristics having the most serious effect on communication performance, because fading reduces the amplitude of a received signal by several dB or several tens of dB. Fading can be overcome by several diversity techniques. A conventional CDMA technique adopts a rake receiver for receiving diverse signals using the delay spread of a channel. A rake receiver performs a diversity reception technique for receiving a multi-path signal. However, the diversity reception technique does not operate when a delay spread is low.
A time diversity system using interleaving and coding is used in a Doppler spread channel. The time diversity system, however, is not suitable for a low-speed Doppler channel. In a room channel with a high delay spread and a pedestrian channel corresponding to a low Doppler channel, a space diversity system is used to overcome fading. A space diversity system uses at least two antennas. If a signal received via one antenna is attenuated by fading, the space diversity system receives the signal via another antenna. The space diversity system is classified into a reception antenna diversity system using a reception antenna and a transmission antenna diversity system using a transmission antenna. As it is difficult for a mobile station to install the reception antenna diversity system in view of size and costs, it is recommended that a base station use the transmission antenna diversity system.
In the transmission antenna diversity system, there are a closed loop transmission diversity system getting feedback of a downlink channel information from a mobile station to the base station and an open loop transmission diversity system getting no feedback from a mobile station to the base station. In the transmission antenna diversity system, a mobile station searches for an optimal weighted value by measuring the phase and magnitude of a channel. In order to measure the phase and magnitude of a channel, a base station must send different pilot signals for different antennas. A mobile station measures the magnitude and phase of a channel through the pilot signals and searches for an optimal weighted value from the measured channel magnitude and phase information.
Additionally, in the transmission antenna diversity system, if the number of antennas increases, the diversity effect and the signal-to-noise ratio improve. However, the amount of improvement in the diversity effect decreases with an increase in the number of antennas or signal transmission paths used in a base station, that is, with an increase in the degree of diversity. Accordingly, to improve the diversity effect by using a number of antennas is not always preferable. Hence, it is preferable that the number of antennas used in a base station increases to minimize the power of an interference signal and maximize the signal-to-noise ratio of an internal signal, instead of improving the diversity effect.
A transmission adaptive antenna array system developed in consideration of a beamforming effect that minimizes the influence that interference and noise, as well as diversity effect, have upon an internal signal is referred to as a downlink beamforming system. A system using feedback information like a transmission diversity system is referred to as a closed loop downlink beamforming system. The closed loop downlink beamforming system, which uses information fed back from a mobile station to a base station, may degrade the performance of communications by failing to properly reflect changes in channel information if a feedback channel does not have a sufficient bandwidth.
European IMT-2000 standardization group, a 3rd Generation Partnership Project (3GPP) R (Release) 99 version, adopts first and second transmission antenna array (TxAA) modes as a closed loop transmission diversity system for two antennas. Here, the first TxAA mode, which has been proposed by Nokia, feeds only the phase difference between two antennas back. The second TxAA mode, which has been proposed by Motorola, feeds the gains of two antennas as well as their phases back. The first and second TxAA modes are disclosed in the specification set by the 3GPP, a standardization group for a Universal Mobile Telecommunications System (UMTS), which is a European IMT-2000 standard.
The first or second TxAA mode of a closed loop transmission diversity system uses adaptive array antennas and is designed to apply weighted values corresponding to different complex values to the respective adaptive transmission array antennas. The weighted values applied to the adaptive array antennas relate to a transmission channel and can be, for example, w=h* (w and h are vectors). Hereinafter, bold characters indicate vectors, and non-bold characters indicate scalars. Here, h denotes a transmission array channel and w denotes a transmission array antenna weighted value vector.
Among mobile communication systems, a system using a frequency division duplex (FDD) generally has a transmission channel and a reception channel that have different characteristics from each other, and accordingly must feed transmission channel information back in order to inform a base station of the transmission channel h. To do this, the first or second TxAA mode is designed so that a mobile station obtains the information on the weighted value w to be obtained from the channel information on the channel h and sends the obtained weighted value information to the base station. The first TxAA mode quantizes only a θ2−θ1 part corresponding to a phase component from the information on the weighted value w (=|w1|exp(jθ1), |w2|exp(jθ2)], where w1 and w2 denotes scalars) into two bits and feeds the two bits back. Accordingly, the precision of a phase is π/2, and a quantization error is π/4 at the maximum. In order to increase the efficiency of the feedback, the first TxAA mode uses a refining method of updating only one bit out of two feedback bits every moment. For example, a combination of two bits can be {b(2k), b(2k−1)} or {b(2k), b(2k+1)}, where b denotes a bit fed back in units of slots every moment. The second TxAA mode feeds back both a phase and a gain, which are the components of the weighted value information. The phase is fed back three (3) bits at a time, and the gain is fed back one (1) bit at a time. Accordingly, the precision of the phase is π/4 and a quantization error is π/8 at the maximum. In order to increase the efficiency of the feedback, the second TxAA mode uses a progressive refining mode for updating only one bit out of the four feedback bits every moment. A refining mode provides each bit having an orthogonal basis value, while the progressive refining mode does not set the value of each bit.
The above-described first and second TxAA modes have the following problems when the number of antennas and the characteristics of a space-time channel vary.
If the number of antennas increases, a weighted value for each antenna must be fed back, and hence a significant amount of information to be fed back is created. Thus, depending on the movement speed of a mobile station, the first and second TxAA modes degrade the communication performance. That is, generally, if the movement speed of a mobile station increases in a fading channel, a change in the space-time channel becomes of considerable concern. Thus, the feedback speed of channel information must increase. However, if the feedback speed is limited, feedback information increasing with an increase in the number of antennas consequently degrades the performance of communications.
If the distance between antennas is not sufficient, the correlation between channels in each antenna increases. If the correlation between channels increases, the information amount of a channel matrix decreases. The effective use of a feedback method prevents performance degradation in a high-speed moving body environment even if the number of antennas increases. However, since the first and second TxAA modes are constructed under the assumption that the channels of two antennas that constitute the space-time channels are completely independent from each other, they cannot be used effectively when the number of antennas and the characteristics of the space-time channel change. In addition, the first and second TxAA modes have never been applied to an environment using more than 2 antennas and cannot provide excellent performance even when using 3 or more antennas.